Negative photoresists are at present used to a great extent in the production of electric circuits and electronic components. In particular, the structuring of the conductor or semiconductor substrates by photographic techniques in the production of highly and very highly integrated semiconductor circuits is effected with the aid of photoresists.
The customary negative photoresist compositions of today contain, as a solution in an organic solvent, cyclized rubber, as the essential component, and a photosensitive bisazido compound which acts as a photo-crosslinking agent.
According to customary procedures, the substrate surfaces are coated with the photoresist composition by known methods and the solvent is removed by predrying at temperatures of about 100.degree. C. The photoresist coating is then exposed to, in general, ultraviolet radiation imagewise through a negative mask, whereupon crosslinking of the polymer material is caused, in accordance with the image pattern, in the areas of the coating exposed to the radiation by the radiation-sensitive bisazido compound. The non-exposed parts of the coating are removed by treatment with a solvent, which serves as a developer, the insoluble areas of the image crosslinked by radiation remaining on the substrate surface as a relief structure.
The high degree of miniaturization, which is still progressing further, of electronic components produced with the aid of photoresist techniques demands an extremely high quality of the photoresist compositions used, especially in respect to the resolving power to be achieved and of the adhesion of the resist coating to the substrate.
However, negative photoresists according to the state of the art do not adequately fulfill these quality requirements. Thus, for example, when such photoresists are used on substrates which have a surface of high reflecting power, which is as a rule the case with semiconductor materials and metals, such as, for example, silicon, aluminum, copper, chromium and the like--the resolving power of the photoresists during irradiation is considerably reduced by reflection phenomena occurring on the surface of the substrate. As a result of irregular scattering of the incident light on the substrate surface, the scattered light penetrates into areas of the resist coating which, according to the image pattern of the negative mask, should not be exposed to radiation. The crosslinking of the polymer material which is also triggered off in these areas results, after development, in relief structures with an increased line width and reduced edge sharpness. Due to these effects, fine and very fine structures such as are to be produced in microelectronics can be obtained only with a limited fidelity of reproduction.
Attempts to solve the problem described are known.
Thus, for example, Japanese Auslegeschrift No. 37562/76 proposes the addition of radiation-absorbing dyestuffs, in particular 4-N,N-diethylaminoazobenzene, to photoresist compositions based on cyclized rubber and photosensitive bisazido compounds to reduce these adverse effects.
However, the compounds mentioned there as radiation absorbers have the disadvantage that they have a great tendency to sublime at temperatures up to 100.degree. C., such as are required during predrying to achieve good adhesion of the resist coating to the substrate. The concentration of these substances in the photoresist coating is thereby reduced to such a degree that a high, reproducible resolution cannot be achieved under these conditions.
U.S. Pat. No. 4,268,603 describes negative photoresist compositions based on cyclized rubber and bisazido compounds to which a mixture of radiation-absorbing substances which emit fluorescence is added to avoid the adverse effects described. Aminoazobenzene derivatives, inter alia the compound 4-N-ethyl-N-(2-hydroxyethyl)-aminoazobenzene, are mentioned here as radiation absorbers. These photoresists of relatively complicated composition are said to ensure high fidelity of reproduction of the resist image.
It has been found, however, that the radiation absorbers used are not sufficiently soluble in the photoresist formulations in order to be present in the resist coating in an adequately effective concentration. It has furthermore been found that some of the dissolved dyestuff crystallizes out as solid substance in the coating during coating of the substrates and subsequent drying.
Such resist materials accordingly still have disadvantages in respect of current requirements.